1. Field of the Invention
The present invention relates to a method for making an electroluminescent lamp, and more particularly, to a method for making an electroluminescent lamp wherein the electroluminescent active layer is made by ultraviolet (“UV”) light curing.
2. Background Art
A typical electroluminescent lamp is a multilayer thin film structure that emits visible light when activated by an applied voltage. Such electroluminescent lamps have found utility in a number of different applications, such as, backlighting for cell phones, backlighting for hand held personal organizers, lighting for point of purchase displays, and backlighting for automotive dash boards.
The active layer (hereafter referred to as the “electroluminescent active layer”) is the layer in an electroluminescent lamp which actually emits the light produced by such a device. The electroluminescent active layer will often contain a phosphor. This electroluminescent active layer has previously been deposited by applying various curable compositions to a substrate. However, the compositions typically contain organic solvents that do not incorporate into the electroluminescent active layer after curing. Such solvent-based systems are undesirable because of the hazards and expenses associated with volatile organic solvents. Furthermore, these solvent-containing compositions often produce layers marred by defects and decreased film quality.
In a typical electroluminescent lamp, the active layer comprises one layer of a multilayer electroluminescent device. An example of such a device would contain a substrate made of polycarbonate or glass coated with a transparent conductor such as fluorine doped tin oxide or indium tin oxide. Metallic grid lines are patterned onto the substrate. The active layer is then applied by screen printing the electroluminescent composition onto the substrate with gridlines. A dielectric coating is then optionally applied over the structure. Finally, the device is coated with a metallic backing. The active layer in such electroluminescent devices typically contains a phosphor. Such various oxide or nitride phosphors may or may not be encapsulated with silica. Encapsulation protects the phosphor from deleterious environmental effects.
Electroluminescent lamps in addition to the electroluminescent active layer often include an opaque conductive layer, a dielectric layer, and a transparent conductive layer. Typically, such layers have been made by a number of thin film processes, which include, chemical vapor deposition, thermal curing, sputtering, thermal dip coating, and evaporation. Fabrication of an electroluminescent lamp where each layer is fabricated by a single type of process is desirable.
Many thin layers, such as the electroluminescent active layer used in an electroluminescent lamp, are often made from compositions that are cured by heat. These heat curable compositions require the use of organic solvents that contain a significant amount of volatile organic compounds (VOCs). These VOCs escape into the atmosphere while the heat curable composition dries. Such solvent-based systems are undesirable because of the hazards and expenses associated with VOCs. The hazards include water and air pollution and the expenses include the cost of complying with strict government regulation on solvent emission levels. In contrast, UV curable compositions contain reactive monomers instead of solvents, thus eliminating the detrimental effects of the VOCs.
The use of heat curable compositions not only raises environmental concerns but other disadvantages exist with their use as well. Heat curable compositions suffer from slow cure times that lead to decreased productivity. These compositions require high energy for curing due to energy loss as well as the energy required to heat the substrate. Additionally, many heat curable compositions yield poor film properties that result in decreased value of the end product.
UV curing is an alternative to heat curing. UV curable compositions are typically applied to a substrate through spraying, screen printing, dipping or brushing for the protection or decoration of the substrate. In the usual application, a substrate such as metals, glass, or plastics is coated with the composition and then UV light is introduced to complete the curing process. The UV curable compositions offer many advantages over typical heat curable compositions.